Testing the upper use temperature of perfluoroelastomers
The market for industrial-use elastomers is defined by extremes. Whether chemical compatibility, plasma resistance, high and low temperatures, or other characteristics, engineers demand the highest possible performance.
By Dr Thomas Reger, Senior Scientist at Greene, Tweed
Greater performance allows manufacturers to build processes or products that give them a competitive advantage through innovation or efficiency. Notably, interest in upper use temperature, an important elastomer characteristic for many applications, continues to increase. Current high-end perfluoroelastomer upper use temperatures hover between 300 and 320°C, and customers are looking for materials that push this limit even higher.
The challenge to elastomer vendors is to not only develop materials that offer higher service temperatures, but also reliably measure those characteristics to the satisfaction of customers. Long-term compression set testing offers an effective data-driven method for establishing the upper use temperatures of novel FKM and FFKM materials.
Market pressure and historical practice
Demand for higher service temperatures exists in many industries. High pressure, high temperature applications in oil and gas exploration and processing, for example, motivate interest in elastomeric seals capable of withstanding high temperature and high loads. The semiconductor industry also has a strong interest in temperature performance. The continued miniaturisation of chips has driven the development of new processes for patterning smaller features. These processes may incorporate aggressive chemistries and/or higher temperatures.
Therefore, the sealing elements in the manufacturing equipment must be compatible with the process or they could degrade. This may result in loss of sealing force and potentially release foreign particles onto the chips, leading to undesirable defects. So the value of higher performance seals is realised by both minimising equipment downtime and increasing yield due to fewer defects.
When choosing an elastomer for an application, the standard question, “’What is the maximum service temperature of this compound?’ often receives the same answer from application engineers: It depends. What is the application? How long will it be exposed to that temperature? Will it undergo temperature cycling? These are important questions for ensuring the right fit. Fundamentally, though, their answer will revolve around the temperature range established for a material.
Historically, the upper use temperature has been established in a variety of ways. It is sometimes based on a recommendation from the polymer raw material supplier or in some cases by analogy to an earlier, related compound. For established elastomer formulations, the accuracy of the service temperature can be judged by performance feedback gleaned from customers.
For newly developed compounds, these methods for estimation are potentially inexact. Customers rightfully expect a scientific, data-driven determination of a material’s temperature capability. To accommodate this, a novel application of a common testing procedure delivers reliable information about upper use temperature.
The effects of cross-linking chemistry on temperature characteristics
Relative to hydrocarbon rubber materials, fluoroelastomers (FKMs) provide increased thermal stability, chemical resistance and comparable mechanical properties. Perfluoroelastomers (FFKMs) offered the next evolution in high performance materials with the highest temperature capability and broadest chemical resistance of any rubber material.
They are used for sealing in the most aggressive environments, including gas turbines in aircraft engines, processing pumps in chemical plants, down-hole drilling equipment for oil and gas exploration, and chamber seals in semiconductor processing equipment.
Cross-linking of polymer chains gives rubber compounds their elastomeric properties. How a particular elastomer is cross-linked has a major effect on the upper use temperature of that material. Examination of the polymer composition and cross-link structure of fluoroelastomers and perfluoroelastomers aids in understanding the properties and performance of these materials.
The fully-fluorinated carbon backbone of FFKM elastomers is the principal reason for their excellent chemical resistance and high thermal stability. Incorporation of a cure-site monomer and subsequent cross-linking (or curing) of perfluoropolymer chains confers elasticity and allows the material to return to near its original shape after being under compression. There are two general classes for curing of FFKMs: peroxide and nitrile.
FKMs may also be peroxide-cured if an appropriate cure-site monomer has been included in the base polymer. The cure process of both the peroxide and nitrile-cure class connects individual polymer chains through reactions taking place between the cure-site monomer and a curative and/or catalyst. The curative is a separate component that is usually incorporated during the compounding process.
Curative selection, in addition to base polymer choice, is also critical for achieving optimal material performance. To demonstrate the compression set testing methodology, Greene, Tweed’s Advanced Technology Group evaluated an FKM compound along with three FFKM compounds, all having different cross-link chemistries.
The compression set testing methodology
Long-term compression set testing is one method for establishing upper use temperature of a material. Compression set represents the tendency of an elastomer to undergo permanent deformation when subjected to a compressive load at temperature.
A material that resists compression set, therefore, is able to recover to close to its original cross-section after the compressive load is removed. This is an important property for an elastomer compound with the goal to maintain sealing force through its service history.
Temperature is the most significant contributor to compression set increase of an elastomer in application. Over time, the chemical bonds in the compound can degrade and the material will begin to lose elasticity. This process is accelerated as the temperature is increased.
By measuring the compression set of an elastomer at various temperatures across multiple time-points, an estimation of the upper use temperature can be made. We have defined the upper use as the temperature at which a seal will have 80% compression set after 1,000 hours. While this represents a long term (6 weeks) continuous use temperature, a material may also be able to handle shorter excursions (< 1 week) to higher temperatures.
This methodology has a number of benefits for customers. These tests can be conducted relatively easily in an experimental lab setting, so no large capital investment in equipment is passed on to customers. Additionally, compression set is a routine test for evaluating any new material—this method simply extends its utility to establish upper use temperature. More importantly, in typical seal applications, a material is under compression for an extended period of time. Evaluating temperature characteristics in the context of pressure gives a better approximation of field performance than evaluating thermal properties without material compression.
The results show there is a clear dependence of the upper use temperature on the elastomer class, cure system, and curative selection. Consistent with known service performance, the nitrile-cure FFKM material exhibited the highest temperature capability and the peroxide-cure FKM showed the lowest.
Within the peroxide-cure FFKM class, a bis-olefin curative proved superior to TAIC, which may be due to the higher amount of alkyl carbon-hydrogen bonds that are incorporated in the cross-link structure with TAIC. Materials containing more carbon-hydrogen bonds (weaker than carbon-fluorine bonds) tend to have the lowest upper use temperature.
Compression set testing establishes temperature characteristics for superior elastomer R&D
The results demonstrate that compression set testing can be used to reliably establish the upper use temperature of both FKM and FFKM elastomers. With this methodology, Greene, Tweed research and development is bolstered by an effective tool to differentiate between otherwise similar compounds to formulate elastomer compounds with consistently superior temperature characteristics.
Furthermore, this test provides valuable data to customers in the semiconductor industry and others interested in understanding material properties to a higher level of reliability before incorporating a compound into process or product.
Ultimately, compression set testing of elastomers to determine upper use temperature will enable Greene, Tweed customers to innovate with elastomers that perform at higher temperature and provide greater confidence in the capabilities of the materials they depend upon.